U.S. patent application number 16/944206 was filed with the patent office on 2022-02-03 for testing of led devices during pick and place operations.
The applicant listed for this patent is ASM Technology Singapore Pte Ltd. Invention is credited to Shun Yan LEE, Ka Yee MAK, Gary Peter WIDDOWSON, Sai Kit WONG, Chi Wah YUEN.
Application Number | 20220037213 16/944206 |
Document ID | / |
Family ID | |
Filed Date | 2022-02-03 |
United States Patent
Application |
20220037213 |
Kind Code |
A1 |
LEE; Shun Yan ; et
al. |
February 3, 2022 |
TESTING OF LED DEVICES DURING PICK AND PLACE OPERATIONS
Abstract
A pick and place LED testing apparatus, comprising: a test
station operative in use to power a group of LEDs; a bondhead
operative in use to pick said group of LEDs from a source wafer and
place said group of LEDs on said test station for testing; and an
optical sensor operative in use to measure an optical
characteristic of said group of LEDs when tested, wherein at least
a portion of said bondhead is translucent to provide an optical
path from said group of LEDs to said optical sensor.
Inventors: |
LEE; Shun Yan; (Hong Kong,
HK) ; WONG; Sai Kit; (Hong Kong, HK) ; YUEN;
Chi Wah; (Hong Kong, HK) ; MAK; Ka Yee; (Hong
Kong, HK) ; WIDDOWSON; Gary Peter; (Hong Kong,
HK) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ASM Technology Singapore Pte Ltd |
Singapore |
|
SG |
|
|
Appl. No.: |
16/944206 |
Filed: |
July 31, 2020 |
International
Class: |
H01L 21/66 20060101
H01L021/66; H01L 33/00 20060101 H01L033/00 |
Claims
1. A pick and place LED testing apparatus, comprising: a test
station operative in use to power a group of LEDs; a bond head
operative in use to pick said group of LEDs from a source wafer and
place said group of LEDs on said test station for testing; and an
optical sensor operative in use to measure an optical
characteristic of said group of LEDs during said testing, wherein
at least a portion of said bond head allows light to pass at least
partially through the bond head to provide an optical path from
said group of LEDs to said optical sensor.
2. The apparatus of claim 1, wherein said portion of said bond head
is translucent.
3. The apparatus of claim 1, wherein said portion of said bond head
comprises a translucent holder operative in use to hold said group
of LEDs such that the optical path passes at least partially
through the translucent holder.
4. The apparatus of claim 3, wherein said translucent holder
comprises a plurality of protrusions extending from a surface of
said bond head towards said group of LEDs.
5. The apparatus of claim 4, wherein said plurality of protrusions
comprise an array of protrusions, each protrusion being spaced from
another protrusion by an integer multiple of a pitch between LEDs
from the source wafer.
6. The apparatus of claim 3, wherein said translucent holder is
elastic and provides for contact adhesion between said translucent
holder and at least one LED from said group of LEDs.
7. The apparatus of claim 3, wherein said translucent holder is
operative in use to pick up said group of LEDs by contacting a
major light-emitting surface of each LED.
8. The apparatus of claim 3, wherein said translucent holder is
operative in use to pick up said group of LEDs by contacting a
surface of each LED other than a surface of each LED having
electrical contacts.
9. The apparatus of claim 1, wherein said portion of said bond head
comprises an optical assembly configured to redirect light emitted
by each LED from a major light-emitting surface of each LED in an
emission direction towards said bond head to a redirected direction
which is transverse to said emission direction.
10. The apparatus of claim 9, wherein said optical sensor is
mounted on a side of said bond head to align with said redirected
direction.
11. The apparatus of claim 1, wherein said test station comprises a
group of electrical contacts operative in use to power said group
of LEDs during testing, and said bond head is operative in use to
place said group of LEDs on said test station aligned with said
group of electric contacts to power said group of LEDs when
testing.
12. The apparatus of claim 1, comprising an electrical tester
operative in use to measure electrical characteristics of said
group of LEDs on said test station when testing.
13. The apparatus of claim 1, wherein said optical sensor is
operative in use to identify defective LEDs within said group of
LEDs which fail to achieve at least one of a selected threshold of
a measured optical characteristic and a selected threshold of a
measured electrical characteristic.
14. The apparatus of claim 1, further comprising a plurality of bin
carriers, each bin carrier being operative to receive said group of
LEDs from the bond head based on said measured optical
characteristics, wherein a mixture of LEDs of different colour
coordinates within each bin carrier share similar measured optical
characteristics.
15. The apparatus of claim 1, further comprising a plurality of bin
carriers, each bin carrier being operative to receive said group of
LEDs from the bond head based on said measured optical
characteristics, wherein a mixture of LEDs of different colour
coordinates within each bin carrier fall within a range of measured
optical characteristics for that bin carrier.
16. The apparatus of claim 1, further comprising a gang bonding
carrier operative to receive groups of LEDs from the same bin
carrier in a predetermined arrangement comprising a mixture of LEDs
of different colour coordinates, and to bond such groups of LEDs
onto an LED display system in the predetermined arrangement
17. The apparatus of claim 1, further comprising a tool operative
in use to perform at least one of a removal and a replacement of
defective LEDs that are received on the gang bonding carrier.
18. The apparatus of claim 1, wherein said optical characteristic
comprises at least one of an intensity and a colour coordinate of
said group of LEDs.
19. The apparatus of claim 18, wherein said intensity comprises an
average intensity of said group of LEDs and said colour coordinate
comprises an average colour coordinate of said group of LEDs.
20. A method, comprising: picking a group of LEDs from a source
wafer using a bond head and placing said group of LEDs on a test
station for testing; powering said group of LEDs on said test
station; and measuring an optical characteristic of said group of
LEDs using an optical sensor, at least a portion of said bond head
allowing light to pass at least partially through the bond head to
provide an optical path from said group of LEDs to an optical
sensor.
Description
FIELD OF THE INVENTION
[0001] The invention relates to an apparatus and method for testing
light emitting diode (LED) devices during pick and place
operations.
BACKGROUND
[0002] Techniques for testing LEDs exist. Such techniques typically
pick an LED and then place it against an electrical circuit to
provide electrical power to perform optical and/or electrical
characteristic testing in order to verify the characteristics and
operation of that LED. However, apparatus for performing such
testing can be slow, complex and less compact than is
desirable.
[0003] It would be beneficial to provide an arrangement which is
faster, simpler and more compact as compared to the prior art.
SUMMARY OF THE INVENTION
[0004] It is thus an object of this invention to seek to provide an
arrangement which overcomes at least some of the aforementioned
problems of the prior art.
[0005] According to a first aspect of the present invention, there
is provided a pick and place LED testing apparatus, comprising: a
test station operative in use to power a group of LEDs; a bondhead
operative in use to pick said group of LEDs from a source wafer and
place said group of LEDs on said test station for testing; and an
optical sensor operative in use to measure an optical
characteristic of said group of LEDs when tested, wherein at least
a portion of said bondhead is translucent to provide an optical
path from said group of LEDs to said optical sensor.
[0006] Accordingly, a pick and place apparatus is provided. The
apparatus may be for LED testing. The apparatus may comprise a test
station or testbed which may power or activate a group of LEDs. The
group of LEDs may comprise a plurality of LEDs. The apparatus may
comprise a bond head which may pick the group of LEDs from a wafer
and place that group of LEDs onto the test station to be tested.
The apparatus may comprise an optical sensor which measures the
optical characteristics of the group of LEDs on the test station.
The bond head may be configured to provide an optical path which
allows light from the LEDs to pass through the bond head to the
optical sensor. In this way, the optical sensor can be located on
the same side of the LEDs as the bond head, which reduces the
complexity of the apparatus and provides for a more compact
apparatus. Also, by testing groups of LEDs, the speed of the
apparatus is improved.
[0007] The portion of the bond head may be translucent. That is to
say, the portion of the bond head may allow for transmission of
light therethrough.
[0008] The portion of the bond head may comprise a translucent
holder which holds the group of LEDs such that the optical path
passes at least partially through the translucent holder. That is
to say, the translucent holder forms part of the optical path
between the LEDs and the optical sensor.
[0009] The translucent holder may comprise a plurality of
protrusions extending from a surface of the bond head towards the
group of LEDs. Typically, a single protrusion is provided for a
corresponding single LED within the group of LEDs.
[0010] The plurality of protrusions may comprise an array of
protrusions, each protrusion being spaced from another protrusion
by an integer multiple of a pitch between LEDs from the source
wafer. Accordingly, the array of protrusions may be configured such
that every nth LED is contacted by a single, corresponding
protrusion.
[0011] The translucent holder may be elastic and provide for
contact adhesion between the translucent holder and at least one
LED from the group of LEDs. Typically, a single protrusion adheres
to a single corresponding LED from the group of LEDs.
[0012] The translucent holder may pick up the group of LEDs by
contacting with a major light emitting surface of each LED within
the group.
[0013] The translucent holder may pick up the group of LEDs by
contacting with a surface of each LED other than a surface having
electrical contacts. In other words, the translucent holder
performs contact adhesion with a surface of the LEDs which fails to
have electrical contacts. This leaves the electrical contacts
exposed for electrical contact with the test station.
[0014] The portion of the bond head may comprise an optical
assembly which redirects light emitted by each LED from a major
light emitting surface of each LED in an emission direction towards
the bond head to a redirected direction which is transverse to the
emission direction. In other words, the optical assembly alters the
optical path in a direction which is transverse to the main
emission direction of light from the LEDs.
[0015] The optical sensor may be mounted on a side of the bond head
to align with the redirected direction.
[0016] The test station may comprise a group of electrical contacts
which power the group of LEDs during testing. The bond head may
place the group of LEDs on the test station aligned with the group
of electrical contacts to power the group of LEDs during
testing.
[0017] The electrical contacts may be spaced apart by the integer
multiple of the pitch between the LEDs from the source wafer. In
other words, the contacts are located to be positionally aligned
with the electrical contacts on the LEDs held by the bond head.
[0018] The integer multiple may be 3 or a multiple of 3.
[0019] The group of electrical contacts may face towards the bond
head.
[0020] The bond head may place the group of LEDs on the test
station aligned with the group of electrical contacts to power the
group of LEDs during testing.
[0021] The apparatus may comprise an electrical tester which
measures electrical characteristics of the group of LEDs on the
test station during testing. Hence, the electrical tester may
measure the voltage and/or current of the group of LEDs, subgroups
of the LEDs and/or individual LEDs during testing.
[0022] The optical sensor may identify defective LEDs within the
group of LEDs as those which fail to achieve a selected threshold
of a measured optical characteristic and/or a selected threshold of
a measured electrical characteristic.
[0023] The apparatus may comprise a controller which controls the
bond head to place the group of LEDs on a bin carrier based on or
in response to the measured optical characteristics. Hence, the
measured optical characteristics determine the bin carrier onto
which the group of LEDs is placed.
[0024] The apparatus may comprise a plurality of bin carriers. Each
bin carrier may receive a group of LEDs from the bond head based on
or in response to the measured optical characteristics. A mixture
of LEDs of different colour coordinates within each bin carrier may
share similar measured optical characteristics. Hence, each bin
carrier may carry LEDs of different colour coordinates but each
group of LEDs within the bin carrier may have similar optical
characteristics.
[0025] The apparatus may comprise a plurality of bin carriers. Each
bin carrier may receive the group of LEDs from the bond head based
on or responsive to the measured optical characteristics. A mixture
of LEDs of different colour coordinates within each bin carrier may
fall within a range of measured optical characteristics for that
bin carrier. Hence, each bin carrier may carry LEDs of different
colour coordinates but each group of LEDs within the bin carrier
may fall within the same range of optical characteristics.
[0026] The apparatus may comprise a gang bonding carrier which
receives groups of LEDs from the same bin carrier in a
predetermined or selected arrangement which comprises a mixture of
LEDs of different colour coordinates. The gang bond carrier may be
used to bond such groups of LEDs onto an LED display system in a
selected or predetermined arrangement of bonding.
[0027] The apparatus may comprise a tool which performs a removal
and/or a replacement of defective LEDs that are received on the
gang bonding carrier.
[0028] The optical characteristic may comprise an intensity and/or
a colour coordinate of the group of LEDs.
[0029] The intensity may comprise an average intensity of the group
of LEDs and the colour coordinate may comprise an average colour
coordinate of the group of LEDs.
[0030] According to a second aspect, there is provided a method,
comprising: picking a group of LEDs from a source wafer using a
bond head and placing said group of LEDs on a test station for
testing; powering said group of LEDs on said test station; and
measuring an optical characteristic of said group of LEDs using an
optical sensor, at least a portion of said bond head allowing light
to pass at least partially through the bond head to provide an
optical path from said group of LEDs to an optical sensor.
[0031] The portion of said bond head may be translucent.
[0032] The portion of said bond head may comprises a translucent
holder and said picking may comprise holding said group of LEDs
with said translucent holder such that the optical path passes at
least partially through the translucent holder.
[0033] The translucent holder may comprise a plurality of
protrusions extending from a surface of said bond head towards said
group of LEDs.
[0034] The plurality of protrusions may comprise an array of
protrusions, each protrusion may be spaced from another protrusion
by an integer multiple of a pitch between LEDs from the source
wafer.
[0035] The translucent holder may be elastic and may provide for
contact adhesion between said translucent holder and at least one
LED from said group of LEDs.
[0036] The picking may comprise picking up said group of LEDs by
contacting a major light-emitting surface of each LED with said
translucent holder.
[0037] The picking may comprise picking up said group of LEDs by
contacting a surface of each LED other than a surface of each LED
having electrical contacts with said translucent holder.
[0038] The method may comprise redirecting light emitted by each
LED from the major light-emitting surface of each LED in an
emission direction towards said bond head to a redirected direction
which is transverse to said emission direction using said portion
of said bond head comprising an optical assembly.
[0039] The optical sensor may be mounted on a side of said bond
head to align with said redirected direction.
[0040] The placing may comprise placing said group of LEDs on said
test station aligned with a group of electric contacts provided by
said test station and said powering may comprise powering said
group of LEDs with said group of electrical contacts.
[0041] The method may comprise measuring electrical characteristics
of said group of LEDs on said test station with an electrical
tester.
[0042] The measuring may comprise identifying defective LEDs within
said group of LEDs which fail to achieve at least one of a selected
threshold of said measured optical characteristics and a selected
threshold of said measured electrical characteristics.
[0043] The method may comprise receiving said group of LEDs from
the bond head onto one of a plurality of bin carriers based on said
measured optical characteristics, wherein a mixture of LEDs of
different colour coordinates within each bin carrier share similar
measured optical characteristics.
[0044] The method may comprise receiving said group of LEDs from
the bond head onto one of a plurality of bin carriers based on said
measured optical characteristics, wherein a mixture of LEDs of
different colour coordinates within each bin carrier fall within a
range of measured optical characteristics for that bin carrier.
[0045] The method may comprise receiving groups of LEDs comprising
a mixture of LEDs of different colour coordinates from the same bin
carrier in a predetermined arrangement onto a gang bonding carrier
and bonding such groups of LEDs onto an LED display system in the
predetermined arrangement.
[0046] The method may comprise performing a removal and/or a
replacement of defective LEDs that are received on the gang bonding
carrier using a tool.
[0047] The optical characteristics may comprise at least one of an
intensity and a colour coordinate of said group of LEDs.
[0048] The intensity may comprise an average intensity of said
group of LEDs and said colour coordinate comprises an average
colour coordinate of said group of LEDs.
[0049] These and other features, aspects, and advantages will
become better understood with regard to the description section,
appended claims, and accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0050] Embodiments of the present invention will now be described,
by way of example only, with reference to the accompanying
drawings, in which:
[0051] FIG. 1 illustrates schematically a pick and place LED
testing apparatus according to one embodiment;
[0052] FIG. 2A is an isometric schematic view of the bond head;
[0053] FIG. 2B is a schematic cross-section through the bond
head;
[0054] FIG. 3 is a partial exploded view of components attached to
the optical assembly enclosure;
[0055] FIG. 4A and FIG. 4B illustrate schematically the test board
in more detail;
[0056] FIG. 5 illustrates schematically a selective
interconnect;
[0057] FIG. 6 shows schematically the group of LEDs positioned onto
the electrodes using a stamp and columns for testing;
[0058] FIG. 7 shows an exemplary layout of a test board 80;
[0059] FIG. 8 is a flowchart illustrating the main steps for
performing LED characterization;
[0060] FIG. 9 illustrates powering columns of LEDs;
[0061] FIG. 10 illustrates placing a group of LEDs in the
appropriate bin carrier;
[0062] FIG. 11 illustrates the placing of different groups of LEDs
onto a bin carrier;
[0063] FIG. 12 illustrates the main processing steps for populating
gang bond carriers;
[0064] FIG. 13 illustrates populating the gang bond carriers in
more detail; and
[0065] FIG. 14 illustrates the removal of defective LEDs from a
gang bond carrier prior to bonding the LEDs.
[0066] In the drawings, like parts are denoted by like reference
numerals.
DETAILED DESCRIPTION
[0067] Before discussing the embodiments in any more detail, first
an overview will be provided. Some embodiments provide a pick and
place LED testing apparatus which has a bond head configured to
allow light from a group of LEDs to pass at least partially through
it during testing. This enables groups of LEDs to be picked from a
source wafer, placed on a test board for optical, and optionally
electrical, testing. The optical, and optionally electrical,
characteristics of that group of LEDs are determined and the LEDs
are then placed on one of a plurality of different bin carriers
depending on an average optical, and optionally electrical,
characteristic of that group of LEDs. This means that each of the
different bin carriers fills with groups of LEDs having similar
optical characteristics. An example of an optical characteristic is
a colour coordinate of an LED, wherein each value in a colour
coordinate represents a position of a colour in a colour space,
such as different shades of a certain colour.
[0068] In some embodiments, each bin carrier is filled with groups
of LEDs of different colour coordinates which share similar average
optical characteristics. This means that each bin carrier contains
LEDs of different colour coordinates which have similar optical
characteristics. This allows different groups of LEDs with varying
colour coordinates within a bin to be picked and placed onto a gang
bonding carrier for use in forming a colour display with varying
colour coordinates, and enables each display to be formed from LEDs
of different colour coordinates, each sharing similar optical
characteristics to provide for a more uniform display. Furthermore,
this approach can help address problems with existing optical
testing approaches which can be inconvenient in production, or
cannot distinguish between LED problems, substrate problems, or
bonding problems, and which may lead to incorrect correlation. The
results may also not be precise enough to conduct sorting (and so
may affect the colour consistency of the display). This may also
lead to situations of over-rejection of good LEDs or passing of
defective LEDs. In addition, this approach can help address
problems with existing electrical testing approaches which can be
limited to LEDs with sapphire material and of sizes of at least 90
um, because of the limitation of probe pin sizes and minimum
pin-to-pin distances. Further, pin contact stresses can be too
large and may damage the electrodes of the microLEDs and would be
suitable only for lateral type LEDs or flip chip type LEDs.
Pick and Place Apparatus
[0069] FIG. 1 illustrates schematically a pick and place LED
testing apparatus 10 according to one embodiment. A source wafer
station 20 is provided which has a plurality of source wafers 30
thereon. Each source wafer 30 may contain an array of LEDs, such as
micro-LEDs in the form of flip-chip type micro-LEDs. X actuators 40
and Y actuators 50 enable the source wafer station 20 to be moved
along X and Y axes with respect to a bond head 60. A pedestal 70 is
provided which carries a test board 80 and a plurality of bin
carriers 90. The X actuators 40 and the Y actuators 50 enable the
pedestal 70 to be moved along the X and Y axes with respect to the
bond head 60. A tray 100 is provided which carries a plurality of
gang bond carriers 115. The tray 100 is moveable with respect to
the bond head 60 along the X and Y axes. The bond head 60 is also
moveable with respect to the source wafer station 20, the pedestal
70 and the tray 100 along a Z axis, which is transverse to the X
and Y axes. A removal tool 105 is provided which removes and
optionally replaces defective LEDs from the gang bond carriers 115,
as will be explained in more detail below.
Bond Head
[0070] FIG. 2A is an isometric schematic view of the bond head 60
and FIG. 2B is a schematic cross-section through the bond head 60.
Extending from a main body 65 of the bond head 60 is an optical
assembly enclosure 110 (see FIG. 3). Extending from the optical
assembly enclosure 110 is a collet 120. Extending from the collet
120 is a base 130. Extending from the base 130 is a stamp 140.
Within the optical assembly enclosure 110 is provided a prism 150
and a lens assembly 160. Attached to the optical assembly enclosure
110 is an optical sensor 170 such as a charge coupled device or a
colour meter.
[0071] As will be explained in more detail below, the stamp 140,
the base 130 and the collet 120 provide an optical path for light
emitted from LEDs under test along an optical axis 180. The optical
axis 180 is redirected by the prism 150 through the lens assembly
160 and onto the optical sensor 170. Hence, it can be seen that the
bond head 60 provides an optical path which allows light to pass at
least partially through the bond head 60 from LEDs under test to
the optical sensor 170 in the optical assembly enclosure 110.
[0072] FIG. 3 is a partial exploded view of components attached to
the optical assembly enclosure 110. As can be seen, the optical
assembly enclosure 110 has a vacuum groove 190 which receives the
collet 120. The vacuum groove 190 is in fluid communication with
vacuum holes 200 which, when a vacuum is applied, holds the collet
120 in place against the optical assembly enclosure 110. Further
vacuum holes 210 in the optical assembly enclosure 110 align with
vacuum holes 220 in the collet 120. When a vacuum is applied, this
also holds the base 130 against the collet 120.
[0073] As can be seen in FIG. 3, the stamp 140 has an array of
columns 230 protruding from an exposed major face of the stamp 140.
Each column 230 is dimensioned to match generally the dimensions of
the micro-LEDs to be tested. Also, the distance or pitch between
adjacent columns 230 is selected to be an integer multiple of the
distance or pitch between LEDs from the source wafer 30. In this
example, the columns 230 are positioned to pick every third LED
from the source wafer 30, but it will be appreciated that different
relative spacings may be provided. Also in this example, although
it is illustrated that the size of the group of LEDs picked by the
columns 230 is 56 LEDs, it should be appreciated that many more
columns would be provided in practice in order to increase the
number of LEDs to be picked up.
[0074] The columns 230, the stamp 140, the base 130 and the collet
120 are each translucent or transparent to allow light from the
LEDs to pass through an aperture 240 in the optical assembly
enclosure 110 to be conveyed to the optical sensor 170. Typically,
the columns 230 and stamp 140 are formed from polydimethylsiloxane
(PMDS). Typically, the base 130 and the collet 120 are formed from
glass, quartz or sapphire glass.
Test Board
[0075] FIG. 4A and FIG. 4B illustrate schematically the test board
80 in more detail. As can be seen in FIG. 4B, LEDs 260 attached to
the columns 230 are placed on the test board 80 and the columns
230, the stamp 140, the base 130 and the collet 120 provide an
optical path for a light cone 250 from each LED 260 to project
along the optical axis 180 into the optical assembly enclosure 110
and onto the optical sensor 170.
[0076] As can be seen in more detail in FIG. 4A, each LED 260 has
electrodes 270 which make electrical contact with electrodes 280 on
the exposed surface of the test board 80 which faces towards the
bond head 60. Typically, the test board 80 is formed from a glass
with the electrodes 280 printed on its surface. As the bond head 60
moves along the Z axis with respect to the test board 80,
electrical contact between the electrodes 270 and the electrodes
280 occurs to enable the LEDs 260 to be powered and the light cones
250 to be generated. A slight compression from the elasticity of
the columns 230 limit the pressure experienced by the LEDs 260 and
provide for more reliable electrical connection with between the
electrodes 270 and 280. This allows the optical sensor 170 to
perform optical measurements of, for example, intensity,
wavelength, colour (coordinate), flux and optical defect inspection
for these measurements to be recorded for each LED 260 and for
average values to be calculated and recorded for the group of LEDs
260.
[0077] FIG. 5 illustrates schematically a selective interconnect
which can be used to power individual pairs of electrodes 280 so
that individual LEDs 260 or sub groups of LEDs within the group of
LEDs 260 placed on the test board 80 can be selectively powered as
required through the use of switches 290, 300. A controller 310
controls the state of the switches 290, 300 and supplies a voltage
across the pairs of electrodes 280. The electrical characteristics
such as forward voltage and reverse current of the powered LEDs 260
can then be recorded.
[0078] FIG. 6 shows schematically the group of LEDs 260 positioned
onto the electrodes 280 by the stamp 140 and columns 230 for
testing.
[0079] FIG. 7 shows an exemplary layout of a test board 80'
configured to receive a group of nine LEDs formed in a 3.times.3
array. The LEDs 260 contact with electrodes 280'. The electrodes
280' are coupled with either a ground trace layer 285B' or a V+
trace layer 285A' which are printed on the surface of the test
board 80' and separated by an insulating layer 283'.
LED Characterization
[0080] FIG. 8 is a flowchart illustrating the main steps when
performing LED characterization. At step S10, the bond head 60 is
located above the source wafer station 20, over the array or group
of LEDs 260 comprised in the source wafer 30 to be tested. The bond
head 60 moves along the Z axis in the direction of the source wafer
30 and the columns 230 overlying the group of LEDs 260 press
against those LEDs 260. The tackiness of the columns 230 causes the
LEDs to adhere to the columns 230 and enables the LEDs 260 to be
removed from the source wafer 30 as the bond head 60 moves away
from the source wafer 30 along the Z axis.
[0081] At step S20, the bond head 60 is moved along the Z axis away
from the source wafer 30 and the bond head 60 moves along the X and
Y axes to overlie electrodes 280 on the test board 80. The bond
head then moves along the Z axis towards the test board 80 and the
electrodes 270 contact the electrodes 280.
[0082] At step S30, the controller 310 and switches 290, 300 power
to the group or sub groups of LEDs 260 on the test board 80. For
example, as shown in FIG. 9 in [1] a first column of LEDs 260 are
powered.
[0083] At steps S40, S50 and S60, the first column of LEDs 260 are
powered such that individual light spots displayed on the optical
sensor 170 have essentially no overlap. A power supply provides a
constant current to the column of LEDs 260. A forward voltage (Vf)
and a reverse current (Ir) across a circuit of each LED 260 is
measured and recorded. Further, selected optical outputs (such as
intensity and colour coordinates of each LED 260) measured by the
optical sensor 170 is also measured and recorded for each LED 260.
Thereafter, the controller 310 and the switches 290, 300 power the
next column of LEDs, and the aforesaid parameters of voltage,
reverse current and optical outputs of the LEDs in that column as
shown in [2] are measured and recorded. Finally the controller 310
and the switches 290, 300 power the next column of LEDs to measure
and record the aforesaid parameters for LEDs in that column as
shown in [3].
[0084] An apparatus controller (not shown) is pre-programmed with
characteristics or characteristic ranges to be stored by each bin
carrier 90 at step S70.
[0085] An average electrical characteristic and optical
characteristic for the complete group of LEDs 260 is compared
against the bin carrier ranges at step S80.
[0086] At step S90, the group of LEDs 260 is then placed in the
appropriate bin carrier 90 for that average electrical and optical
characteristic, as illustrated in FIG. 10.
[0087] Should the average of the group of LEDs 260 fail to achieve
a minimum expected threshold, then the group of LEDs 260 is placed
in a no-good bin carrier at step S100.
[0088] At step S110, a map file for the bin carrier 90 that
received the group of LEDs 260 is updated to indicate where the
group of LEDs 260 have been placed within that bin carrier 90, and
processing returns to step S10 to pick and characterize the next
group of LEDs 260 from one of the wafers 30 until all the LEDs 260
have been picked, characterized and placed in an appropriate bin
carrier 90.
[0089] FIG. 11 illustrates the placing of different groups of LEDs
260 onto a bin carrier 90. As can be seen, different groups of LEDs
260 from different wafers 30 are picked, tested and from their
characteristics it is determined that they should be placed in the
bin carrier 90 designated as bin 1. Accordingly, the groups of LEDs
260 from different source wafers 30 are placed in locations within
bin 1 and the map file will indicate the locations of these groups
of LEDs 260, together with the colour coordinate or which source
wafer 30 those groups of LEDs 260 are from. Accordingly, it can be
seen that each bin carrier 90 gradually gets filled with groups of
different coloured LEDs 260, each sharing similar
characteristics.
Gang Bond Carrier
[0090] FIG. 12 illustrates the main processing steps for populating
the gang bond carriers 110.
[0091] At step S110, as mentioned above, the different bin carriers
90 each contain groups of LEDs 260 with similar
characteristics.
[0092] At step S120, the bond head 60 is moved along the X and Y
axes with respect to the pedestal 70 to overlie the intended bin
carrier 90 which carries LEDs 260 with the required
characteristics.
[0093] At step S130, the map of groups of LEDs within that bin
carrier 90 is loaded. The apparatus controller selects an initial
group of LEDs using the map file by locating the bond head 60 over
that group of LEDs 260 and moving the bond head 60 along the Z axis
towards the bin carrier 90 so that the columns 230 contact the LEDs
260 which then adhere to the columns 230.
[0094] At step S140, the bond head 60 moves along the Z axis away
from the bin carrier 90 to remove the group of LEDs 260 from the
bin carrier 90. The bond head 60 is moved with respect to the tray
100 along the X and Y axes to overlie the intended gang bond
carrier 115. The bond head 60 then moves along the Z axis towards
the gang bond carrier 115 to place the group of LEDs 260 onto the
gang bond carrier 115 at the required position.
[0095] At step S150, a map for the gang bond carrier 115 is then
generated to reflect the position of the group of LEDs 260 that
have been placed on that gang bond carrier 115.
[0096] FIG. 13 illustrates this process in more detail. As can be
seen, initially groups of red LEDs 260 are selected from the bin
carrier 90 designated as bin 1 and are placed on the gang bond
carrier 115 at spaced intervals. Thereafter, groups of green LEDs
260 are selected from the bin carrier 90 designated as bin 1 and
placed on the gang bond carrier 115 at a slightly offset position
from the already placed red LEDs 260. Groups of blue LEDs 260 are
then selected and from the bin carrier 90 designated as bin 1 and
are placed on the gang bond carrier 115 at slightly offset
positions. In this way, it can be seen that the gang bond carrier
115 is populated with red, green and blue LEDs each sharing similar
or grouped characteristics in order to provide for a uniform
display. It will be appreciated that although FIG. 13 illustrates
populating the gang bond carrier 115 with single colours in a
particular order, it will be appreciated that this need not be the
case and that any groups of LEDs 260 from within the same carrier
can be placed at any locations within the gang bond carrier 115
until all locations within the gang bond carrier 115 have been
occupied.
[0097] Returning now to FIG. 12, at step S160, the removal tool 105
optionally removes and/or replaces any defective LEDs, specifically
no-good and/or defective LEDs 260, from the gang bond carrier 115,
as illustrated in FIG. 14 by reference to the map file. Those LEDs
260 may be simply omitted or replacement individual LEDs 260 added,
if required. Once a gang bond carrier 115 is complete, then at step
170, a complete array of arranged LEDs placed on the gang bond
carrier 115 is then removed for bonding at the same time to create
the display.
[0098] Hence, it can be seen that in some embodiments, an elastic
stamp is used to pick microLEDs to avoid damage during pick up. The
elastic stamp may be made from AD300 to pick up and place LEDs onto
a glass substrate to avoid damage during put down. The test board
is made of glass, and the electrodes are made by electroplating
them onto the glass substrate, and so are substantially flat, which
has a reduced size concern and less stress concentration for
avoiding damage. The electroluminescence (EL) test directly
measures the electrical and optical properties of each LED die
precisely without any need for correlation with other
characteristics. The light from each microLED is directly captured
and analysed, and sorting is further possible when optical
properties of an individual die are known, which is desirable for
display makers to fulfil industry standards of colour and intensity
or brightness. Mixing is possible when optical properties of
individual LED devices are known, which is desirable for a display
maker to fulfil industry standards of colour and intensity or
brightness, and for filtering out the no-good LEDs, which can
dramatically improve the yield during the manufacture of display
panels.
[0099] Although the present invention has been described in
considerable detail with reference to certain embodiments, other
embodiments are possible.
[0100] Therefore, the spirit and scope of the appended claims
should not be limited to the description of the embodiments
contained herein.
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